1. Field of the Invention
The present invention relates to a magnetic head used in a floppy disc drive (FDD) or the like.
2. Description of the Related Art
An example of a conventional magnetic head is shown in FIG. 6 (Japanese Laid-Open Patent No. Hei 7-235013).
In FIG. 6, the magnetic head 1 is mainly composed of a front core 2 which is slidably moved to a magnetic recording medium (not shown) on one side of the front core 2, a back yoke 3 made of a magnetic material such as Mn--Zn ferrite material, Ni--Zn ferrite material or the like adhered by adhesives such as epoxy resin or UV ray curing resin (UV resin) on the other side of the front core 2, a read-write coil 4 and erasing coil 5 to be fitted in the back yoke 3.
The front core 2 is mainly composed of a composite core 8 (core front member) forming a read-write gap 6 and erasing gap 7, and a first outrigger 9 and a second outrigger 10 which are adhered by epoxy resin or the like on both sides of the composite core 8 to be substantially in the form of a rectangular shape.
The composite core 8 is mainly composed of a first core member 11 having a read-write gap 6 on one side, and a second core member 12 to be bonded to the first core member 11 and having an erasing gap 7.
The first core member 11 is mainly composed of a first center core 13 made of magnetic material extending to the other side of the front core 2, and a read-write side core on the front side 14 made of magnetic material extending to the other side of the front core 2 and bonded to the first center core 13 through the read-write gap 6.
The second core member 12 is mainly composed of a second center core 15 made of magnetic material bonded to the first center core 13 and extending to the other side of the front core 2, and an erasing side core on the front side 16 made of magnetic material, extending to the other side of the front core 2 and bonded to the second center core 15 through the erasing gap 7. The second core member 12 is set at the same height of that of the first core member 11. Incidentally, in FIG. 6, reference numeral 17 denotes a glass for the purpose of magnetic insulation.
The first outrigger 9 is in the form of a plate with the same length as that of the composite core and is provided with a groove 18 extending in a longitudinal direction on one side. The second outrigger 10 is in the form of a rectangular parallelepiped shape with the same longitudinal length as that of the composite core 8.
The back yoke 3 is mainly composed of first, second and third leg portions 19, 20 and 21, a closure member 22 having substantially parallelepiped annular member for covering the erasing coil and the read-write coils 5 and 4 and a base plate 23 having a substantially rectangular shape and connected to the base end of the closure member 22 and to the base ends of the first, second and third leg portions 19, 20 and 21.
In this case, only side portions 23a and 23b of the base plate 23 confronting each other in a widthwise direction (hereinafter referred to as first and second side portions) are connected to the closure member 22 (the parts connected to the first and second side portions 23a and 23b will hereinafter be referred to the first and second side wall portions 22a and 22b). Gaps 24 and 24 are formed between in space of third and fourth side portions 23c and 23d perpendicular to the first and second side portions 23a and 23b, respectively, and in space of side wall portions 22c and 22d of the closure member 22 (hereinafter referred to as to third and fourth side wall portions, respectively), so that the lead lines 4a of the read-write coil 4 and lead lines 5a of the erasing coil 5 may be extracted to the outside therefrom. Also, the upper surface portion of the first side wall portion 22a is bonded to the first outrigger 9.
The first and second leg portions 19 and 20 are formed along the third and fourth side portions 23c and 23d and in confronting relation with each other. The third leg portion 21 is formed between the first and second leg portions 19 and 20. The read-write coil 4 and the erasing coil 5 are fitted to the first an second leg portions 19 and 20, respectively.
The first and second leg portions 19 and 20 are bonded to the read-write side core 14 and erasing side core 16 on the front side, respectively. And the third leg portion 21 is bonded to the first center core 13 and the second center core 15. Further, the first side wall portion 22a is bonded with the first outrigger 9. The epoxy resin or UV resin (ultraviolet ray curing resin) is used for adhesion so that the front core 2 and the back yoke 3 is formed in one body.
Then, as described above, the front core 2 and the back yoke 3 are formed in one body so that the read-write magnetic circuit (not identified by numeral) is formed of the first center core 13, the third leg portion 21, the base plate 23, the first leg portion 19, the read-write side core on the front side 14 and the read-write gap 6. Also, in the same manner, the erasing magnetic circuit (not identified by numeral) is formed of the second center core 15, the third leg portion 21, the base plate 23, the second leg portion 20, the erasing side core on the front side 16 and the erasing gap 7. Incidentally, the third leg portion 21 forms a common magnetic path of the read-write and the erasing magnetic circuit.
The lead lines 4a and 5a of the read-write and erasing coils 4 and 5 are inserted into the gaps 24 and 24 of the back yoke 3 and extracted to the outside.
Now, since the relative speed to the magnetic recording medium of the magnetic head used for the FDD is low, it is necessary to increase the number of turns of the coil for the purpose of obtaining the desired reproduction output. In order to increase the number of turns of the oil, it is necessary to thin and elongate the leg portions (first, second and third leg portions 19, 20 and 21) of the back yoke 3 within a range where its efficiency is not deteriorated.
On one hand, in case of a ferrite sintered material such as Mn--Zn, in order to enhance the formation precision, the amount of binder to be contained may be increased. According to the above, the containment rate of the magnetic material for a unit volume is reduced, which cause the magnetic reluctance to be increased.
On the other hand, when the amount of the binder to be contained is reduced, the mechanical strength after molding and before sintering is reduced, so that, there is a tendency that the damage of the sintered products is remarkable during the conveyor delivery in the production steps. For this reason, in the magnetic head 1 described in the above prior art of which the back yoke is made of the ferrite sintered material such as Mn--Zn, it is difficult to reduce a thickness of each of the leg portions in the back yoke 3 (first, second and third leg portions 19, 20 and 21) to a predetermined level, and to increase a length thereof beyond a predetermined level. Accordingly, it is impossible in practical to increase the number of the turns of the coil, so that the predetermined reproduction output is not obtained. Also, since the gaps 24 and 24 are formed on the bottom side of the back yoke 3 (in the space between the third side portion 23c and the third side wall portion 22c and the space between the fourth side portion 23d and the fourth side wall portion 22d), and the lead lines 4a and 5a of the read-write and erasing coils 4 and 5 can be inserted into the gaps 24 and 24 and extracted to the outside therefrom. However, by providing the gaps 24 and 24, the structure of the back yoke 3 is complicated. Therefore, when the ferrite such as Mn--Zn is molded and sintered in the above manner, not only is it difficult to obtain the predetermined dimension but also the magnetic flux of the external magnetic field is introduced from the gaps 24 and 24 formed on the bottom side of the back yoke 3 into the interior to thereby reduce the shield effect of the magnetic head 1.